Generally, a liquid crystal display device (LCD) controls light transmittance of a liquid crystal having a dielectric anisotropy using an electric field to thereby display a picture. To this end, the LCD includes a liquid crystal display panel for displaying a picture by a liquid crystal cell matrix, and a driving circuit for driving the liquid crystal display panel.
Referring to FIG. 1, a related art liquid crystal display panel is comprised of a color filter substrate 10 and the thin film transistor substrate 20 that are joined to each other with a liquid crystal 24 therebetween.
The color filter substrate 10 includes a black matrix 4, a color filter 6 and a common electrode 8 that are sequentially provided on an upper glass substrate 2. The black matrix 4 is provided in a matrix type on the upper glass substrate 2. The black matrix 4 divides an area of the upper glass substrate 2 into a plurality of cell areas to be provided with the color filter 6, and prevents a light interference between adjacent cells and an external light reflection. The color filter 6 is provided at the cell areas divided by the black matrix 4 in such a manner to be divided into red(R), green(G) and blue(B) ones, thereby transmitting red, green and blue lights. The common electrode 8 is formed from a transparent conductive layer entirely coated onto the color filter 6, and supplies a common voltage Vcom that serves as a reference voltage upon driving of the liquid crystal 24. Further, an over-coated layer (not shown) for smoothing the color filter 6 may be provided between the color filter 6 and the common electrode 8.
The thin film transistor substrate 20 includes a thin film transistor 18 and a pixel electrode 22 provided for each cell area defined by an intersection between a gate line 14 and a data line 16 at a lower glass substrate 12. The thin film transistor 18 applies a data signal from the data line 16 to the pixel electrode 22 in response to a gate signal from the gate line 14. The pixel electrode 22 formed from a transparent conductive layer supplies a data signal from the thin film transistor 18 to drive the liquid crystal 24.
The liquid crystal 24 having a dielectric anisotropy is rotated in accordance with an electric field formed by a data signal from a pixel electrode 22 and a common voltage Vcom from the common electrode 8 to control light transmittance to implement a gray scale level.
Further, the liquid crystal display panel includes an alignment film for initial aligning the liquid crystal 24, and a spacer (not shown) for constantly keeping a cell gap between the color filter substrate 10 and the thin film transistor substrate 20.
In such the liquid crystal display panel, the color filter substrate 10 and the thin film transistor substrate 20 are formed by a plurality of mask processes. Herein, one mask process includes a lot of processes such as thin film deposition (coating), cleaning, photolithography, etching, photo-resist stripping and inspection processes, etc.
Particularly, since the thin film transistor substrate includes the semiconductor process and requires the plurality of mask processes, it has a complicate fabricating process to act as a major factor in the manufacturing cost rise of the liquid crystal display panel. Therefore, the thin film transistor substrate has been developed toward a reduction in the number of mask process from a five-round mask process that is a standard mask process.
Meanwhile, the liquid crystal display devices are largely classified into a vertical electric field applying type and a horizontal electric field applying type depending upon a direction of the electric field driving the liquid crystal.
The liquid crystal display device of vertical electric field applying type drives a liquid crystal in a twisted nematic (TN) mode with a vertical electric field formed between a pixel electrode and a common electrode arranged in opposition to each other on the upper and lower substrate. The liquid crystal display device of vertical electric field applying type has an advantage of a large aperture ratio while having a drawback of a narrow viewing angle about 90°.
The liquid crystal display device of horizontal electric field applying type drives a liquid crystal in an in-plane switching (IPS) mode with a horizontal electric field between the pixel electrode and the common electrode arranged in parallel to each other on the lower substrate. The liquid crystal display device of horizontal electric field applying type has an advantage of a wide viewing angle about 160°.
But, in the liquid crystal display device of horizontal electric field applying type, the pixel electrode and the common electrode are formed from a plurality of finger images at each pixel area, so that the liquid crystal display device of horizontal electric field applying type has a drawback of a small aperture ratio. A line width of the pixel electrode and the common electrode must be decreased in order to increase the aperture ratio, but it is limited by an exposure resolution in the photolithography process.
FIG. 2A to FIG. 2C are sectional views showing a method of forming a related art transparent electrode.
Referring to FIG. 2A, a transparent conductive layer 42 is formed on a substrate 40, and a photo-resist pattern 44 is formed on the transparent conductive layer 42. In this case, it is difficult to have a minimum line width of the photo-resist pattern 44 much smaller than the exposure resolution of an exposure element. For example, when a photo resolution of a scan type is approximately 4 μm, it is impossible that a minimum line width of the photo-resist pattern 44 is smaller than 4 μm.
And, referring to FIG. 2B and FIG. 2C, the transparent conductive layer 42 is etched by the etching process, thereby providing a transparent electrode 46 overlapped with the photo-resist pattern 44, and the photo-resist pattern 44 is removed by the stripping process. In this case, the transparent electrode 46 is formed in such a manner as to have a narrower line width than the photo-resist pattern 44 by an etching CD (Critical Dimension) bias A, but when the minimum line width of the photo-resist pattern 44 is 4 μm, it is impossible that a line width of the electrode 46 is less than 3 μm.
Thus, the minimum line width of the pixel electrode and the common electrode of the liquid crystal display device of horizontal electric field applying type is limited by the exposure resolution. Thus, there is limit to improve the aperture ratio.